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cosmos-sdk/docs/sdk/core/multistore.md

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MultiStore

TODO: reconcile this with everything ... would be nice to have this explanation somewhere but where does it belong ? So far we've already showed how to use it all by creating KVStore keys and calling app.MountStoresIAVL !

The Cosmos-SDK provides a special Merkle database called a MultiStore to be used for all application storage. The MultiStore consists of multiple Stores that must be mounted to the MultiStore during application setup. Stores are mounted to the MultiStore using a capabilities key, ensuring that only parts of the program with access to the key can access the store.

The goals of the MultiStore are as follows:

  • Enforce separation of concerns at the storage level

  • Restrict access to storage using capabilities

  • Support multiple Store implementations in a single MultiStore, for instance the Tendermint IAVL tree and the Ethereum Patricia Trie

  • Merkle proofs for various queries (existence, absence, range, etc.) on current and retained historical state

  • Allow for iteration within Stores

  • Provide caching for intermediate state during execution of blocks and transactions (including for iteration)

  • Support historical state pruning and snapshotting

Currently, all Stores in the MultiStore must satisfy the KVStore interface, which defines a simple key-value store. In the future, we may support more kinds of stores, such as a HeapStore or a NDStore for multidimensional storage.

Mounting Stores

Stores are mounted during application setup. To mount some stores, first create their capability-keys:

fooKey := sdk.NewKVStoreKey("foo")
barKey := sdk.NewKVStoreKey("bar")
catKey := sdk.NewKVStoreKey("cat")

Stores are mounted directly on the BaseApp. They can either specify their own database, or share the primary one already passed to the BaseApp.

In this example, foo and bar will share the primary database, while cat will specify its own:

catDB := dbm.NewMemDB()
app.MountStore(fooKey, sdk.StoreTypeIAVL)
app.MountStore(barKey, sdk.StoreTypeIAVL)
app.MountStoreWithDB(catKey, sdk.StoreTypeIAVL, catDB)

Accessing Stores

In the Cosmos-SDK, the only way to access a store is with a capability-key. Only modules given explicit access to the capability-key will be able to access the corresponding store. Access to the MultiStore is mediated through the Context.

Notes

TODO: move this to the spec

In the example above, all IAVL nodes (inner and leaf) will be stored in mainDB with the prefix of "s/k:foo/" and "s/k:bar/" respectively, thus sharing the mainDB. All IAVL nodes (inner and leaf) for the cat KVStore are stored separately in catDB with the prefix of "s/_/". The "s/k:KEY/" and "s/_/" prefixes are there to disambiguate store items from other items of non-storage concern.